I'm echoing the response that the most common combination is probably the right one. Determining name Kanji is not an exact science, however. The name might vary based on the region, but Hirata is pretty universal around Japan. Sometimes, though, people change the character spelling legally for any of several reasons. One reason might be luck. If a particularly bad thing happens to a family, they might change the characters. Another reason might be overcoming 'burakumin' status, where people with certain family names from certain places are considered hereditary social outcasts. It's illegal to single people out as burakumin, but it still goes on. Another reason is personal preference. Two very famous transvestite entertainers in Japan changed the first character of their family name from the character for three to the character for beautiful.